Inkjet printing of conductive inks onto flexible or rigid substrates, such as paper, polyester, synthetic polymer sheets, acrylic, polyimide, glass or FR-4, to create printed electronic circuits has made fabricating prototypes and finished electronic products significantly faster than by conventional lithographic etched circuit board fabrication. One of two basic techniques is typically used to deposit the ink onto the substrate: drop-on-demand (DOD), such as from piezo- or bubble-driven jets, or continuous spray, such as a shear thinning ink that is constantly deposited as a viscous ink onto either a substrate or itself to form a free-standing wire. In many contexts, the inks are made electrically conductive by the inclusion of metallic, such as silver, microparticles or nanoparticles.
Once deposited on a substrate, the particles should be sintered, i.e., fused into a solid mass, to provide electrical and mechanical connections between the particles. In some cases, high temperatures, such as about 300° C., are used to sinter the particles. The particles are sintered to consolidate the particles, for mechanical stability, and to form metal-to-metal electrically conductive junctions among the particles, thereby forming an end-to-end electrically conductive line or area.
However, many substrates or components mounted on the substrates are incompatible with such high temperatures. Thus, in other cases, so-called “non-thermal sintering” is used to fuse, bind or otherwise electrically join the conductive particles together. For example, U.S. Pat. No. 7,963,646 to Shlomo Magdassi (“Magdassi”), the entire contents of which are hereby incorporated by reference herein for all purposes, describes ink formulations and inkjet printing techniques. Magdassi prints two layers: a metallic ink layer and a solution of flocculant. Alternatively, Magdassi prints the metallic ink on a substrate that has been pre-treated with the flocculant.
Nevertheless, silver inks are limited in their current carrying capacity, often to less than 1,000 Amps per square centimeter (A/cm2). At the current density of most circuits consisting of copper wiring (“traces”), which is about 10,000 A/cm2, circuits fabricated using silver inks typically degrade after short times, often within weeks of fabrication, as a result of electromigration of silver atoms. Higher current carrying capacities and longer lived electric circuits created with conductive inks would be highly desirable and would enable new applications for printed conductors.